Investigation of Cu(In,Ga)Se₂ thin‐film formation during the multi‐stage co‐evaporation process

Abstract

In order to transfer the potential for the high efficiencies seen for Cu(In,Ga)Se 2 (CIGSe) thin films from co-evaporation processes to cheaper large-scale deposition techniques, a more intricate understanding of the CIGSe growth process for high-quality material is required. Hence, the growth mechanism for chalcopyrite-type thin films when varying the Cu content during a multi-stage deposition process is studied. Break-off experiments help to understand the intermediate growth stages of the thin-film formation. The film structure and morphology are studied by X-ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth-resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu-poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In(Ga) Cu antisite defects. An efficiency of 12.7% for vacancy compound-based devices is obtained.

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

“…39 At first, this seems to be in contradiction to our finding of a higher Na content at the surface of the Cu-rich compared to that of the Cu-poor CIGSe/Mo/PI samples. This discrepancy can be explained taking into account that according to Wei et al 20 the main effect of Na (present in small quantities) is the elimination of In Cu defects, which preferably form in Cu-poor CIGSe absorbers.…”

“…Both series were intentionally deposited Cu-poor with respect to the standard Cu:(In þ Ga):Se ¼ 1:1:2 CIGSe stoichiometry, but the degree of Cu deficiency was varied by adjusting the duration of the third deposition stage. 39 It should be noted, however, that the nominal Cu/(In þ Ga) composition at the end of second deposition stage was approx. 1.15.…”

Real-time observation of Cu2ZnSn(S,Se)4 solar cell absorber layer formation from nanoparticle precursors

“…It is well known from the literature that an intermediate Cu-rich composition during co-evaporation of Cu(In, Ga)Se 2 absorber films is favorable for the formation of a large grained morphology and for high energy conversion efficiencies of resulting solar cells. 35,36 Also for co-evaporated CZTS it was shown that a Cu-rich growth has a positive effect on the formation of large grains. 13,15,37 In these cases, the Cu excess is subsequently removed by etching 13 or by a Cupoor co-evaporation stage 15 to obtain single phased CZTS.…”

Evidence for Cu2–xSe platelets at grain boundaries and within grains in Cu(In,Ga)Se2 thin films

“…[2][3][4] However, a Cu-rich stoichiometry is known to lead to the formation of a secondary phase, Cu 2-x Se (x ¼ 0-0.25), at the absorber surface and at grain boundaries (GBs) within the film. [5][6][7] The secondary phase formed at the surface can be removed by chemical treatments (NaCN, KCN, etc.)…”